Structure and function of RABL3 in paclitaxel resistance (2014-2015)

Ovarian cancer affects more than 21,000 women each year and kills approximately 15,000 women each year in the United States. The primary cause of cancer-related mortality in these women is the acquired resistance to chemotherapy. We think it is important to understand how tumor cells acquire resistance so that we may be able to overcome chemotherapy resistance and provide more effective and durable treatment.

The majority of ovarian cancer patients will experience relapse and die of the disease within five years because tumor cells acquire resistance to chemotherapy. Although tumor-specific alterations, such as BRCA1 and BRCA2 for cisplatin resistance and tubulin isoform mutations for paclitaxel resistance, have been shown to contribute to chemotherapy resistance, additional genetic alterations that contribute to chemotherapy resistance are not well characterized. Therefore, we generated a tumor-derived custom cDNA library from 10 tumor samples collected from ovarian cancer patients with chemotherapy resistance disease. We then performed positive selection genetic screens and identified a rare isoform of RABL3 as a modulator of paclitaxel resistance. This particular isoform is detected in 16% (5/31) primary ovarian tumors and in 63% (7/11) recurrent ovarian tumor samples. The isoform is the result of alternative splicing out of exon 6 that encodes RABL3 without G5 box. Mutations in G5 region in other G-proteins have been shown to lower affinity to GDP resulting in enhanced GTP loading and activation of G-protein. Therefore, exon 6-missing isoform may represent an active small G-protein. In addition, exogenous expression of RABL3 enhances paclitaxel resistance while downregulation of exogenous RABL3 decreases paclitaxel resistance.

Based on these results, we hypothesize that enhanced RABL3 activity, either through the expression of splice-variant isoform or overexpression of wild type transcript, promotes paclitaxel resistance. To address this hypothesis, we will strive to determine the GTP binding affinity of RABL3 variants, determine the structure-function relationship to GTP binding affinity, determine the localization of RABL3 variants and their effect on intracellular paclitaxel accumulation, and determine the proteins that interact with RABL3 and regulate RABL3 signal transduction.